The basic economic advantages that can be realized from automating mask alignment procedures in mask defect inspections have been projected many times in the past but technically and economically attractive solutions have not been set forth. In the prior art the positioning and inspection of such masks has generally been performed by individual operators at microscopes with the operators being trained to manually position the masks and to recognize and measure on a statistical sampling basis different types of geometrical defects in the mask. By using a statistical sampling method less than 100% inspection occurs. The inspection costs are maintained at a tolerable level but the yield of semiconductor devices exposed using such partially inspected masks may be severely reduced from what could be achieved.
Previous attempts to automate mask defect inspections have had limited success primarily because of the difficulty in avoiding erroneous defect indications caused by positional errors between the respective areas on the masks which occur due to stepping errors in the production equipment used to create the mask. Such prior inventions did not provide means for resolving this stepping error from defects or other variations in the masks, especially when multiple, complex mask geometries were to be aligned or inspected.
The present invention resolves or avoids these difficulties and is an automatic alignment and inspection system. This invention relies on the measurement of either transmitted or reflected light, reflected or secondary electrons, or back-scattered currents when a controllable beam is selectively addressed onto the unit being inspected with controlled field alignment and especially on specified utilitarian aspects of the mask which can be used as the alignment target for the controlled field alignment.
The present invention achieves a significant improvement over the prior art for it compares the measured data against the original design data thus obtaining an accuracy of approximately 100% with higher resolution geometries.